Mozilla has begun offering Firefox 3.6 to users running older versions of the open source browser. The offer was the first coordinated invitation to Firefox 3.6 that Mozilla has displayed to users of 2008′s Firefox 3.0 and mid-2009′s Firefox 3.5. As with past update offers, the one for Firefox 3.6 lets users accept the upgrade, postpone the next reminder by 24 hours or decline the invitation. The latter, however, doesn’t necessarily block future offers. Mozilla typically issues multiple upgrade invitations to users who have previously declined, hoping to get everyone onto the newest version.